1. Field of the Invention
The present invention provides designs for basecurves which are designed to promote adhesion of an excess polymer HEMA ring to the basecurve during a demolding operation, such that the excess polymer HEMA ring is removed along with the basecurve during the demolding operation.
The present invention relates generally to basecurve mold designs for maintaining adhesion of an excess polymer HEMA ring to a basecurve in a process for molding cast contact lenses in mold assemblies, each comprising a frontcurve and a spaced basecurve, and wherein a molded lens is formed therebetween. The subject invention pertains generally to an arrangement for producing ophthalmic contact lenses in cast contact lens mold assemblies, and provides for the improved removal of an excess polymer HEMA ring with a basecurve mold during a demolding operation.
The present invention is particularly well suited to molded ophthalmic lenses such as hydrogel contact lenses, although it also has applicability to other small, high-precision ophthalmic lenses such as intraocular lenses.
2. Discussion of the Prior Art
As the ophthalmic lens industry has grown, and in particular the industry related to supplying contact lenses which are provided for periodic frequent replacement, the number of contact lenses required to be produced has increased dramatically. This has spurred manufacturers to strive for automated methods and apparatus which are adaptable to automated practices and consistent performance.
It is generally known in the prior art to make ophthalmic lenses, such as soft hydrogel contact lenses, by molding a monomer or monomer mixture in a mold such as one made from polystyrene or polypropylene.
Examples of this prior art can be found in U.S. Pat. Nos. 5,039,459, 4,889,664 and 4,565,348. These patents discuss therein the requirement for a polystyrene mold in which the materials, chemistry and processes are controlled such that the mold portions do not require undue force to separate by sticking to the lens or to each other. In contrast to the above polystyrene molds, another example is the use of polypropylene or polyethylene molds as described in U.S. Pat. No. 4,121,896.
The mold assembly to mold an ophthalmic contact lens typically includes a lower concave mold portion referred to as a frontcurve and an upper convex mold portion referred to as a basecurve. The concave surface of the lower frontcurve and the convex surface of the upper basecurve define therebetween a mold cavity for a contact lens.
A particular problem in the prior art is that the frontcurve and basecurve molds are usually surrounded by a flange, and the monomer or monomer mixture is supplied in excess to the concave frontcurve mold prior to the assembly of the molds. As the molds are placed together, defining the lens and forming an edge, the excess monomer or monomer mixture is expelled from the mold cavity and rests on or between the flange of one or both mold portions. Upon polymerization, this excess material forms an annular ring around the mold assembly which resists separation of the mold portions during a demolding operation. In such contact lens manufacturing processes, lens defects such as chips and tears as well as missing lenses are believed to result, in part, from difficulties in the demolding operation.
In the demolding process, it is desirable that the excess polymer HEMA ring remain adhered to the basecurve for removal of the excess polymer HEMA ring with the basecurve when the basecurve is demolded and removed, leaving only the cast contact lens in the frontcurve.